Field of the Invention
The invention concerns a method for determining a system frequency in magnetic resonance (MR) imaging, as well as a system frequency determination device and a magnetic resonance system.
Description of the Prior Art
The object of frequency adjustment prior to an MR image data acquisition procedure capture process is a reliable and precise determination of the water frequency, which is also referred to as the system frequency. The so-called water frequency corresponds to the frequency of the protons bound in water. The determination is necessary because, when MR diagnostic data are subsequently acquired, the magnetic field can be modified by the introduced object under examination and depends on the location of the measurement in the MR scanner.
In order to ensure a reliable frequency adjustment, maxima of an acquired spectral distribution of MR signals must be analyzed, correctly determined, and finally assigned to the correct substance, such as fat, water or silicone.
The frequency adjustment is conventionally based on the determination of a cross-correlation between an acquired frequency spectrum and a model having two maxima (water and fat at −3.4 ppm of water). This approach functions reasonably well if exactly two maxima are present in the acquired spectrum. If, however, only a single maximum is determined, there is a special procedure in order to determine whether the single maximum is assigned to the substance fat or the substance water. In this situation, again on the basis of a cross-correlation, small secondary maxima are sought on both sides of the primary maximum. If no clear indication can be found for fat or water, the primary maximum is assigned to the substance water. If a switch has previously been operated on the MR system, the operation of which switch is based on the assumption that the fat dominates the MR imaging, then the primary maximum is assigned to the substance fat.
In addition, in the case of chest examinations, the substance silicone can also occur, which involves a third frequency component (at −4.5 ppm of water) in the measured frequency spectrum. A special mode in which silicone is taken into consideration in a model having two maxima can conventionally also be activated in this case, wherein fat is replaced by silicone. In the case of an unclear detection of a single maximum it is defined that the primary maximum is assigned to the substance silicone.
In the conventional method using cross-correlation, widths of the maxima, amplitude ratios and frequency positions of the maxima of the model are compared with the measured signal or the measured frequency spectrum. On current MR systems having highly optimized numeric programs this process requires approximately 300 ms. If in addition a search for a special single maximum is performed, as has been described above, then even more computing time and run time are needed. If a third maximum is also taken into consideration in the calculations, as in the case of the occurrence of silicone, then the computing time with the conventional approach is in the order of minutes, which is not acceptable.